1. Field of the Invention
The present invention relates to a backlight device used for an automotive dual-view display.
2. Description of the Related Art
Traditional displays provide good quality images for a plurality of observers who watch the display from multiple directions simultaneously. For example, the displays used in the airport, railway stations, arena . . . etc, are prefer to have high brightness at all angles even at a wide view angle.
On the other hand, it also exists a situation that the viewers need to view different images from the same display. For example, in an automobile, it is more convenient when the driver view the car navigation, the passenger on the passenger seat can view other images such as a movie from the same display. In order to fulfill such a demand, a dual-view display has been developed. Such dual-view display shows two different images simultaneously, and each of the images can only be viewed in specific angles.
FIG. 10 shows a conventional dual-view display according to the prior art. Such dual-view display is realized by a liquid crystal panel with a slit portion, e.g. the barrier 713, formed therein, where the liquid crystal panel has respective pixels for the left images and for the right images.
In JP Laid Open No. 2005-44642, a backlight device used for automotive liquid crystal display is disclosed. The structure of the disclosed backlight device is as the following. A lens A is located on a light-guide system, in such a way that the prism rows of the lens A are installed on a surface in contact with the light-guide system, and the prism rows are placed in a direction orthogonal to the long side of the light-guide system equipped with a light source. A lens B was located on the top of the lens A. The prism rows of the lens B, which are orthogonal to the prism rows of the lens A, are located on the opposite surface to the surface which is in contact with the lens A. FIG. 11 shows the luminance directivity characteristic of this backlight device, which shows the luminance is uniform in a range between −35° and +35°, i.e. an angle of 35° relative to the left and right from the front face, respectively. In other words, such liquid crystal display consisting of one screen is designed to display just one single image for the driving seat, the passenger seat and the rear seats.
In contrast to the above, the backlight device used for a dual-view display shows two different images respectively to the driving seat and the passenger seat by a liquid crystal display consisting of one screen. According to the above, the backlight device disclosed by JP Laid Open No. 2005-44642 shows a peak luminance level when looking from the front side, and shows a slightly deteriorated luminance while looking from the left and right sides, which is not suitable for a dual-view display. The backlight device for a dual-view display is required to have the peak luminance level in the directions of the driver seat and passenger seat but a low luminance level in the front center direction in order to reduce optical crosstalk of the left and right images. In addition, it is also necessary to decrease the reflection light from the windscreen, which interferes with the view of the driver during night-drive, since the dual-view display requires a narrow luminance directivity characteristic in a longitudinal direction.
Furthermore, WO/2005/071474 discloses a backlight device for a dual-view or multi-view display. As shown in FIG. 6c of WO/2005/071474, the backlight device is constructed by a pair of prism sheets which are disposed in such a way that the prism structures thereof are facing each other. For such configuration, since the tips of the prisms of one prism sheet are in contact with those of the other, the backlight device has a disadvantage of being easily damaged. In addition, in order to direct the partly-collimated light to the prism sheet, a light-guide capable of transmitting the collimated light is required.